The Edison or common lamp screw base comprises a threaded metal shall, a center contact or eyelet and a molded insulator uniting the shell and eyelet. Such bases have been used with incandescent lamps since the turn of the century and are disclosed in U.S. Pat. No. 774,404--Swan (1904). They come in various sizes ranging from miniature through medium to mogul. They include variants such as the three-contact base used with three-way lamps which comprises a contact ring intermediate the base shell and the eyelet, as disclosed in U.S. Pat. No. 2,519,328--Whitmore et al.
The metal parts of such bases were originally made of brass which is corrosion resistant and easy to solder. After World War II aluminum became much cheaper than brass and during the 1950's it was substituted for brass in the base shells of the medium size bases used with common household incandescent lamps. However aluminum tends to gall, that is fret and wear, more than brass and is not as corrosion resistant; up to now it has not been possible to use aluminum bases with lamps operating at very high temperatures or under adverse ambient conditions. Virtually all high intensity discharge lamps utilize brass or nickel-plated brass screw-in bases. Also the large sizes of incandescent lamps and incandescent lamps used in outdoor signs still utilize brass bases.